In digital data transmission systems, filters are often employed to condition signals at various stages during the data transmission process. Systems which include a modem (modulator/demodulator) permit digital data to be transmitted over analog transmission facilities. Typically in a modem environment, the modulator portion of a first modem converts digital data from a computer to an analog signal which may be transmitted over a transmission medium such as a telecommunications line, and the demodulator portion of a second modem converts the transmitted analog signal back into digital form.
Modulators functionally operate to shape a data signal and combine the resulting shaped signal with a sinusoidal carrier frequency to provide a data bearing information signal operating at the carrier frequency. Shaping is performed to provide the data bearing signal appropriate spectral properties which facilitate transmission. The data bearing carrier signal is typically further filtered and sent via the transmission medium to a demodulator on a second modem which reverses the process by recovering the originally transmitted signal from the carrier frequency.
The shaping function of the modulator is typically provided by a finite impulse response (FIR) filter. The manner and extent of shaping is defined by the transfer function provided by the FIR filter.
Typical hardware implementations of FIR filters involve adders and multipliers. Usually, a plurality n of delay elements is provided, off of each of which is a tap. Data signals applied at the taps are individually multiplied, using a multiplier, by coefficients which weight the tapped lines. The resulting weighted data is added at an adder to provide a weighted sum of the previous n values of the data signal. However, the use of adders and multipliers in any FIR implementation results in a large computational burden for the designer, especially if the FIR filter is realized with a large number of taps.
Accordingly, there is a need for an FIR filter which is easily implemented and which eliminates the excessive computational times of known types of FIR filters. It is therefore an object of the present invention to provide a finite impulse response filter having a fast response time for shaping a digital data pulse train in a digital data transmission system. It is a further object of the invention to provide such a filter in a hardware implementation which does not include any adders or multipliers.